Signal wave modulation



Jan. 2, 1940. y w. R. BENNETT 2,185,684

S I GNAL WAVE MODULAT I ON Filed Nov. 5, 1938 www A TTORNE V PatentedJan. 2, 1940 SIGNALv WAVE. MODULATION William R. ennett, Maplewood, N.J., assignor to Bell Telephone Laboratories, Incorporated, New York, N.Y., a corporation ofv New York Application November 5, 1938, Serial No.238,971

'l Claims.

The presentl invention relates to electrical Wave modulation forpurposes of intelligence transmission, and more specifically to theproduction of a single side-band of a modulated Wave. Direct productionof a single side-band in order to avoid having to filter out theunwanted side-band from a double side-band wave has been proposedheretofore. As far as applicant is aware such direct production of asingle side-band has really involved the production of pairs ofsidebands in a circuit so constructed as to cause one side-band of onepair to cancel out one side-band of the other pair leaving the other twoside-bands to reinforce each other in the final output. Such methodrequires phase shifting and circuit balancing operations of a rathercritical character.

In accordance with the present invention such phase shifting circuitsand circuit balances are obviated and direct production of but a single2D side-band based upon a required carrier frequency is achieved.

In a specificy form of the invention to be disclosed in detail herein,the signal is applied to sets of eld coils, arranged in a circle, toproduce a rotating magnetic field, within which are placed stationarysecondary windings with equal angular displacements relative to oneanother.

` The circuit of utilization is switched from one secondary coil to thenext at a rate widely different from the highest signal frequency and,in general, at a rate which is high compared with the highest signalfrequency. The use of a plurality of secondary coils arranged in thismanner and switched into the outgoing circuit in rapid succession avoidsthe dfflculty that would be involved in using a, single rotating coilrotating at the excessively high speed that would be necessary in thecase, for example, of a speech signal.

d@ The method of the invention secures results which are not entirelysimilar to the effects that would be obtained with a rotating coil butwhich yield as one resultant component wave a single sideband which isunaccompanied by the usual opposite side-band and which is easilyseparable from other output components.

The various features and objects of the invention will be made moreapparent from the following detailed description of a preferredembodiment as illustrated in the accompanying drawing.

In the drawing:

Fig. 1 is a schematic diagram of a preferred type of the apparatus forpracticing the inven- (Cl. Mil-171.5)

tion involving the use of a cathode ray tube with a rotating beam as theswitching means;

Figs. 2, 3 and 4 are diagrams representing carrier`and side-bandrelations on the assumption of different numbers of secondary coils. 5

Referring to Fig. 1, field coils l0, il, I2 and i3 produce a rotatingmagnetic field having the frequency and amplitude characteristics of thespeech or other signal, by virtue of the phase quadrature relationsbetween the currents in the m respective pairs of coils caused by use of-degree phase shifter M. The signal may be of any desired type but isillustrated as originating in microphone l5. 'The phase shifter it maybe. for example, a network such as shown in Hartley Patm ent 1,666,206,April 17, 1928, or a double modulating system making use of a carrierwave shifted 90 degrees for one of the mcdulationaas disclosed in CarpePatent 1,747,160, February 18, 1930. go

1n the field oi the coils lil, il, i2, i3 are positioned a number ofstationary coils it with equal angularspacing. In the drawing only a fewof these coils are shown but it will be understood that the coilsactually extend entirely around the cir- 25 .cle indicated. The numberof coils used may be varied as will appear later on. The terminals ofcoils it are brought to the electrodes 2d of a cathode ray tube 26provided with the usual cathode and beam `forming electrodes and withtwo ao pairs of defiecting plates 2i, 22 for causing the beam 23 torotate over the fixed electrodes 2d. The beam is rotated at the carrierfrequency. The carrier generator 25 is connected to the usualphase-splitting circuit 26, 2l for producing phase :sa quadraturevoltages on plates 2i, 22 to rotate the beam.

The electrodes 2d are preferably coated to make them good emitters ofsecondary electrons. The secondarily emitted electrons are collected bycengg trai electrode til which is at positive potential with respect toelectrodes 2d due to battery 3l. The collector electrode it leads to theprimary of an output transformer 33.which may couple the modulatingsystem to any desired type of 45 load circuit 35 through suitableamplifier stages indicated at 3d.

As mentioned above. a rotatingr coil could be placed in the field of thecoils it, i i i2, i3 and rotated at the carrier frequency and wouldproduce 50 at its output terminals directly a single side-band,

yan upper or lower side-band, depending on the direction of coilrotation with respect to the direction of field rotation. In the case ofspeech the frequency of rotation would need to be excessively high andwould involve mechanical dilculties considering that leads would have'to be brought out from the coil terminals. With such a moving coil, theside-band amplitude would be proportional to the side-band frequencysince the electromotive force developed is a function of rate of changeof ux. This would produce distortion which could be compensated by a.suitable equalizer.

In. accordance with this invention the rotation is transferred from thecoil to the switching mechanism. 'I'he rotating iield induces voltagesin the stationary coils I6 which are a function of the signal amplitudeand frequency. These are sampled in rotation by the switching mechanism.The coll whose voltage is utilized may be considered as occupying atsucceeding instants of time the Same positions that the moving coilwould occupy at the same instants but without having to move in order toreach those positions. The resulting side-band amplitude is proportionalto the signal frequency rather than to the side-band frequency as in thecase of the moving coil. This distortion can be compensated for by asuitable equalizer shown at 36, which may precede amplifier 34 ifdesired.

To obtain a quantitative theory taking into account departures fromideal conditions, let us assume that the rotating field vector isrepresented by' H=Q(i cos qt+j sin qt) (1) where i, i, lc are the unitvectors along the axes of a rectangular coordinate system. Let the axisof a coil in the field make an angle a with the horizontal. 'Ihen a unitvector along the coil axis may be represented in vector notation by a=a(cos a-l-j sin a) The voltage induced across the terminals of the coil isdQS B E- N10 dt (a) applied across the amplifier terminals may beexpressed as EU=hqQlf1p0 Sin (qt-ak) (6) where h=NA108, ak=21r(k-1)/n istheangle which the axis of theV kth coil makes with that of the firstcoil, fk(pt) is a switching function expressing the constant ofproportionality between the voltage across the ampliiier input and thevoltage induced across the4 kth coil as a function of time, and n is thetotal number of stationary coils. The properties of this system as asingle side-band modulator may be exhibited by making the assumptionthat the contacts are identicalV and uniformly traversed with angularvelocity p. Assumptions concerning innitely rapid changes from zero tofinite conductance in the switching process are thereby avoided. Inmathematical language, we require where Substituting Equation 8 inEquation 6 l1. E cm2 sin (10) The first slum, which represents thesignal, is zero except when 'n.=1. The second sum, which represents theupper side-bands, is zero except when m-i-l is zero or a multiple of n.The last sum, whichl represents the lower side-bands, Vis zero exceptwhen m-l is zero or a multiple of n. We thus have upper side-bands ononly the (rn- Dst harmonics of the switching frequency,

r=l, 2, and lower side-bands on only the (rn-|-1)st harmonics of theswitching frequency,

r=0, l, 2, The general form for Eg is thus Vfound to be:

h E.= 592cm-, sm Km-1 p+q1f+0m1 nhqQ 'Ihe frequency spacing between thesingle sideband on the fundamental and the next higher side-band is(n+1) times the switching frequency. For rectangular switching with nolost time,

2 E sm n (12) The solution llholds for like directions of rnc) tation ofeld and switching. If the field and the switching process move inopposite directions, the positions of upper and lower side-bands areinterchanged, the eifect'being shown by changing the sign of q in 11. Aless favorable separation of fundamental side-band and the next higherone is thereby obtained.

To illustrate for particular values of n, we note that with threestationary coils, it is possible to get an output wave containing alower side-band on p, an upper on 2p, lower on 4p, upper on 5p,

etc. Or, by reversing one direction of rotation, we get upper side-bandson p, 4p, 1p, and lower side-bandsV on 222,152), 8p, With four`stationary coils, lower side-bands on p, 5p,

9p, and upper side-bands on 3p, 1p, I Ip, may be produced.

These relations are shown graphically in Figs. 2, 3 and 4 which areself-explanatory.

Z sin [(mp-aatfmn] While thetype of switching or commutating circuitshown is the preferred type, the invention is not in any wise limited inits broader aspects to the use of this or any specific type'. Anysuitable arrangement for connecting the coils I6 into the outgoingcircuit in sufliciently rapid succession may be used. A ring of gastubes arranged to fire in succession as disclosed in King-Ohl Patent N0.2,136,621, dated November 15, 1938 may be used. Another type ofcommutation using a succession of biased rectiers having periodicimpulses supplied to them to counteract the bias voltages at proper timedisplacements as disclosed in E. Peterson application Serial No. 221,-297, filed July 26, 1938, may be used. The scope of the invention isindicated in the claims which follow.

What is claimed is:

1. The method of producing a signal modulated wave comprising producinga rotating field represented by a vector having an amplitudeproportional to the maximum amplitude of the signal and rotating atsignal frequency, producing a response proportional to the component ofsuch vector along each of a succession of angular directions, detectingthese responses successively at a rate different from the frequency ofrotation and combining the detected responses in a common circuit.

2.- The method of producing a signal modulated wave in the case of asignal comprised of a band of frequency components comprising producingfrom the signal a rotating field in spacehavingfrequency components eachof which may be represented by a vector proportional to the maximumamplitude of a corresponding frequency component of the signal rotatingat the frequency of the corresponding signal component, producing fromeach frequency component of the iield a response proportional to thecomponent of thecorresponding l connection with a circuit of utilizationat a rate f high in comparison with the highest signal frequency wherebyside-bands are produced at frequency levels spaced apart by at least thefrequency of switching,v and selecting the desired side-band byfiltering.

4. In a. signaling system, a source' of signal waves, magnetic coilsarranged in a circle and so connected to said source as to producerotating magnetic fields of .the signal frequencies, stationary coils atequally spaced angular positions in said fields, a circuit ofutilization, and means of rapidly switching said coils into connectionwith said circuit in rotation.

5. In a signaling system, a source of signal Waves, means comprising eldcoils and connections thereto from said source for producing a rotatingeld having frequency and amplitude vectors determined by the signal,stationary coils in said field, a cathode ray tube having controlelectrodes, connections from said stationary coils to respective controlelectrodes, means for sweepving a cathode beam over said electrodes at afrequency high in comparison with the highest signal frequency, and anoutput circuit operatively related to said electrodes. l

6.*In a signaling system, means including a source of signal waves andfield windings for producing a rotating magnetic field having frequencyand amplitude characteristics determined by the signal, a plurality ofstationary windings inductively related to said rotating field, acircuit of utilization, and electrical commutating means forsuccessively establishing operative connection between each of saidstationary windings and `said circuit of utilization at a rate high incomparison with the highest signal frequency component.

'1. In a signaling system, a cathode ray tube including'means forproducing a rotating beam rotating at a frequency higher than thehighest signal frequency component to be transmitted, a plurality ofemitter electrodes positioned to be scanned by said beam, a collectorelectrode positioned. to receive secondarily emitted electrons from saidemitterelectrodes, a source of signal waves, means including angularlypositioned coils and phase shifting means for producing a rotatingmagnetic field under control of said signal waves, secondary coilsangularly displaced from one another in said rotating eld, individualconnections from said secondary coils to said emitter electrodes, and acircuit of utilization connected between said collector electrode andacommon terminal of said secondary coils.

WILLIAMR. BENNETT.

